The invention relates to a method and a system for vehicle stabilization for a hybrid vehicle in the event of brake slip of the drive wheels or an increased risk thereof.
In the following statements, (engine) torques which accelerate the vehicle have positive values and (engine) torques which decelerate the vehicle have negative values. However, when reference is made explicitly to a braking torque, engine drag torque or recovery torque, such a braking torque, engine drag torque or recovery torque acts on the vehicle to decelerate it with a positive value.
In conventional vehicles driven purely by an internal combustion engine, in the event of an abrupt release of the accelerator, downshifting or braking, the vehicle stability is ensured by regulation of the engine drag torque. A braking torque which counteracts the propulsion of the vehicle, namely the engine drag torque of the internal combustion engine, acts in such conditions. In particular on a slippery road surface, if this braking torque of the internal combustion engine is so great that the effective torque on the wheels can no longer be completely transmitted to the road and significant brake slip occurs, the engine drag torque regulation is engaged and a higher engine torque is set, so that the wheels are accelerated and the slip is reduced.
In the braked condition with the service brake actuated, a difficult situation prevails, since then the braking torque of the drive unit and the braking torque set by means of the service brakes add up, so that then the maximum force transmission capacity of the tires between the vehicle and the road surface can be more easily exceeded.
Such engine drag torque regulation is also employed in hybrid vehicles, which have an internal combustion engine and an electric engine as drive engines, as well as a clutch between the internal combustion engine and the electric motor. By means of the clutch, the torque of the internal combustion engine can be decoupled from the drive wheels. With the clutch closed, the torque of the internal combustion engine and the torque of the electric motor are superimposed on the electric motor, so that the effective torque on the electric motor results from the torque of the internal combustion engine (the torque set by the internal combustion engine) and the torque of the electric motor (the torque set by the electric motor).
Such a hybrid vehicle with an internal combustion engine and an electric motor and a clutch located between them is described in DE 10 2005 024 359 A1. The drive train of the hybrid vehicle includes an internal combustion engine for generating a driving torque. The apparatus further includes an electric motor which is disposed coaxially around a drive shaft and which with one end of its rotor shaft is connected in series to the internal combustion engine by means of the clutch and the drive shaft and which is connected by the other end of the rotor shaft thereof to a gear unit.
The other object of the invention is to provide an improved method as well as a corresponding system for vehicle stabilization in the event of the presence of a braking torque of the internal combustion engine for a hybrid vehicle.
This and other objects are achieved according to the invention.
A first aspect of the invention relates to vehicle stabilization for a hybrid vehicle in the event of brake slip of the drive wheels or increased risk thereof. In this connection the internal combustion engine provides a negative torque which brakes the vehicle, i.e. the internal combustion engine supplies a braking torque. The braking torque of the internal combustion engine typically corresponds to the drag torque. Furthermore, in this situation the torque set by the electric motor is typically a negative torque, i.e. there is a braking torque of the electric motor, for example a recovery torque (in order, for example, to supply the vehicle electrical system with power and/or to charge the electrical energy storage device). Braking torques of the drive motors which counteract the propulsion of the vehicle occur for example in an overrun mode (i.e. when the accelerator pedal is not actuated) or in the braked state.
In the method it is presupposed that the hybrid vehicle includes, between the internal combustion engine and the electric motor, a clutch by which the torque of the internal combustion engine can be decoupled from the drive wheels. With the clutch engaged, the resulting torque on the electric motor is produced by the torque of the internal combustion engine and the torque of the electric motor. The hybrid vehicle is preferably a hybrid structure in which the internal combustion engine is operatively connected via the drive shaft and the clutch to the electric motor and the electric motor is series connected with one end of its rotor shaft to the internal combustion engine via the clutch and the drive shaft, as is described in DE 10 2005 024 359 A1.
According to the method the presence of a specific vehicle situation with brake slip of the drive wheels is recognized. For this purpose, for example, the wheel slip of the drive wheels is evaluated and if a threshold value for the wheel slip is exceeded the presence of such a vehicle situation is inferred. In addition, in order to ascertain such a situation it may also be a precondition that the driver has braked. Optionally, the recognition of such a situation also depends upon the evaluation of the friction coefficients of the drive wheels. Alternatively, the presence of a situation with increased risk of brake slip can also be recognized without brake slip actually having to be present. It may, for example, be provided that such a situation is present in the braked condition if the driver actuates the brake pedal without the wheel slip being evaluated.
If such a vehicle situation with brake slip or with increased risk thereof is recognized, the clutch between the internal combustion engine and the electric motor is released and the torque of the electric motor is increased, i.e. the torque of the electric motor, which is typically negative before release of the clutch, is increased. The increase of the (typically negative) torque of the electric motor in the direction of positive torque values corresponds to a decrease of the braking torque of the electric motor.
By release of the clutch between the internal combustion engine and the electric motor the braking torque (typically the drag torque) of the internal combustion engine can be quickly decoupled from the drive wheels, so that immediately the amount of negative torque of the internal combustion engine acting on the drive wheels is reduced and thus any wheel slip is decreased. Thus the reaction speed of the method according to the invention is significantly higher than the slow change of the engine torque of the internal combustion engine with a conventional regulation of the engine drag torque.
With the clutch released between the internal combustion engine and the electric motor, however, furthermore the electric motor remains coupled to the drive wheels, so that any braking torque of the electric motor would act on the drive wheels. However, according to the invention the torque of the electric motor is increased, i.e. the braking torque of the electric motor (for example the drag torque or recovery torque of the electric motor) is reduced, so that the braking action of the electric motor on the drive wheels is reduced or even completely eliminated. If need be, a positive driving torque can be actively built up on the drive wheels by means of the electric motor in order to reduce the wheel slip by an active driving torque. By comparison with a conventional regulation of the engine drag torque. not only the release of the clutch but also the change of the engine torque of the electric motor take place significantly more quickly than the change of the engine torque of an internal combustion engine.
For implementation of the method, a control unit for vehicle stabilization, a control unit for engine control and a control unit for transmission control are networked appropriately.
With the method a braked vehicle condition with brake slip (or a risk thereof) is preferably recognized, wherein the vehicle is in a braked condition with the service brake actuated. The service brake is typically a hydraulic brake. The service brake can be actuated by the driver by actuation of the brake pedal or automatically by a driver assistance system, for example by an adaptive cruise control (ACC). The release of the clutch and change of the torque of the electric motor as described above preferably takes place with the precondition of actuation of the service brake. When no braked condition is present, for example when a brake slip is recognized, a conventional regulation of the engine drag torque becomes active.
According to a preferred embodiment, when a vehicle state described above is recognized the clutch is released and an engine torque on the electric motor is approximately zero. In this case, with the clutch released, the torque of the electric motor (i.e. the torque set by the electric motor) is also approximately zero, since the decoupled internal combustion engine no longer has any effect on the torque on the electric motor. Thus in this case the torque on the electric motor no longer has a braking effect on the drive wheels, so that the brake slip is reduced.
In this case it is advantageous if, after release of the clutch and the presence of a torque on the electric motor of approximately zero, it is ascertained whether for vehicle stabilization a positive driving torque is required. This can be recognized, for example, by evaluation of the wheel speed and wheel acceleration behavior of the drive wheels. In the case of a need for a positive driving torque, a positive driving torque is set on the electric motor. In this case, with the clutch released, the set torque of the electric motor (i.e. the torque set by the electric motor) is also positive, since the decoupled internal combustion engine no longer has any effect on the torque on the electric motor, and the torque acting on the electric motor corresponds to the torque set by the electric motor (i.e. the torque of the electric motor).
This two-stage approach—namely, firstly setting a torque of approximately zero and then, after a zero torque is present, setting a positive driving torque, if there is a need for this—offers the advantage that the braking torque on the electric motor can be eliminated very quickly and then only if required, if this is insufficient for stabilization of the vehicle, a corrective intervention is performed by predetermination of a positive torque on the electric motor. Thus, if there is no requirement, a release of the brake or pushing of the vehicle is avoided by a positive torque.
It is also contemplated for a positive torque to be set immediately on the electric motor, without a zero torque having previously been set.
In general it is also contemplated that in a first step a first torque is set on the electric motor and, if required, then in a second step a higher torque than the first torque is set on the electric motor.
A second aspect of the invention relates to a system for vehicle stabilization for a hybrid vehicle having an internal combustion engine and an electric motor as drive motors. The system is configured to recognize the presence of a vehicle condition with brake slip of the drive wheels or increased risk thereof. Furthermore, the system is configured, in response to this, to release the clutch between the internal combustion engine and the electric motor and to increase the torque of the electric motor, i.e. to reduce the braking torque of the electric motor in the event of a negative torque of the electric motor.
The system comprises, for example, a control unit for vehicle stabilization, a control unit for engine control and a control unit for transmission control which are networked appropriately for implementation of the above-mentioned functionalities.
The control units can, for example, be connected to one another by way of a common bus system, for example a LIN bus (Local Interconnect Network), a CAN bus (Controller Area Network) or a FlexRay bus.
According to a preferred configuration of the system, for driving stabilization the control unit is configured to recognize a vehicle condition with brake slip of the drive wheels, and to request release of the clutch and a change of torque in the event of recognition of the vehicle condition in the control unit for engine control. The control unit for engine control is configured to request release of the clutch in the control unit for transmission control if the release of the clutch by the control unit for driving stabilization has been requested in the control unit for engine control.
The above description concerning the method according to the invention according to the first aspect of the invention also apply in a corresponding manner to the system according to the invention according to the second aspect of the invention; advantageous embodiments of the system according to the invention correspond to the described advantageous embodiments of the method according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.